Field effect transistor devices with dopant free channels and back gates

ABSTRACT

A method of forming a back gate transistor device includes forming an open isolation trench in a substrate; forming sidewall spacers in the open isolation trench; and using the open isolation trench to perform a doping operation so as to define a doped well region below a bottom surface of the isolation trench that serves as a back gate conductor, wherein the sidewall spacers prevent contamination of a channel region of the back gate transistor device by dopants.

BACKGROUND

The present invention relates generally to semiconductor devicemanufacturing techniques and, more particularly, to forming improvedback gate field effect transistor (FET) devices by using shallow trenchisolation (STI) regions to introduce back gate dopant material.

Semiconductor-on-insulator (SOI) devices, such as silicon-on-insulatordevices, offer several advantages over more conventional semiconductordevices. For example, SOI devices may have lower power consumptionrequirements than other types of devices that perform similar tasks. SOIdevices may also have lower parasitic capacitances than non-SOI devices.This translates into faster switching times for the resulting circuits.In addition, the phenomenon of latch up, which is often exhibited bycomplementary metal-oxide semiconductor (CMOS) devices, may be avoidedwhen circuit devices are manufactured using SOI fabrication processes.SOI devices are also less susceptible to the adverse effects of ionizingradiation and, therefore, tend to be more reliable in applications whereionizing radiation may cause operation errors.

On the other hand, SOI devices generally suffer from floating bodyeffects. The body of an SOI FET stores electrical charge as a functionof the history of the device, thus changing the body voltage accordinglyand becoming a “floating” body. As such, an SOI FET has thresholdvoltages which are difficult to anticipate and control, and which varyin time. The body charge storage effects result in dynamic sub-thresholdvoltage (sub-Vt) leakage and threshold voltage (Vt) mismatch amonggeometrically identical adjacent devices. Floating body effects in anSOI field effect transistor are particularly a concern in static randomaccess memory (SRAM) cells, where Vt matching is extremely important asoperating voltages continue to be scaled down.

An evolution beyond the standard FET (which has a single top gate thatcontrols the FET channel) is the double-gated FET, in which the channelis confined between a top and a bottom gate. Positioning the channelbetween a top gate and a bottom gate allows for control of the channelby the two gates from both sides of the channel, reducing short channeleffects. Further, a double-gated FET may exhibit higher transconductanceand reduced parasitic capacitance as compared to a single-gated FET. Thepresence of the back gate allows for enhanced on-chip power managementand device tuning. Multiple threshold voltage (Vt) devices may also beachieved on a single IC chip by applying different back biases at theback gates of various devices.

SUMMARY

In an exemplary embodiment, a method of forming a back gate transistordevice includes forming an open isolation trench in a substrate; formingsidewall spacers in the open isolation trench; and using the openisolation trench to perform a doping operation so as to define a dopedwell region below a bottom surface of the isolation trench that servesas a back gate conductor, wherein the sidewall spacers preventcontamination of a channel region of the back gate transistor device bydopants.

In another embodiment, a method of forming a back gate transistor deviceincludes forming a protective pad layer over asemiconductor-on-insulator (SOI) substrate, the SOI substrate having abulk substrate layer, a buried insulator (BOX) layer formed on the bulksubstrate layer, and an active SOI layer formed on the buried insulatorlayer; forming an open isolation trench through the protective padlayer, the active SOI layer, the BOX layer, and into a portion of thebulk substrate layer; forming sidewall spacers in the open isolationtrench; and using the open isolation trench to perform a dopingoperation so as to define a doped well region below a bottom surface ofthe isolation trench and the BOX layer, such that the doped well regionserves as a back gate conductor, wherein the sidewall spacers preventcontamination of the BOX layer and the active SOI layer device bydopants.

In another embodiment, a back gate transistor device includes asemiconductor-on-insulator (SOI) substrate, the SOI substrate having abulk substrate layer, a buried insulator (BOX) layer formed on the bulksubstrate layer, and an active SOI layer formed on the buried insulatorlayer; an isolation trench formed through the active SOI layer, the BOXlayer, and into a portion of the bulk substrate layer; sidewall spacersformed in the open isolation trench; at least a portion of the isolationtrench filled with a dopant source insulator material; and a doped wellregion disposed below a bottom surface of the isolation trench and theBOX layer, the doped well region configured as a back gate conductor,wherein dopants from the doped well region originate from the dopantsource insulator material.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Referring to the exemplary drawings wherein like elements are numberedalike in the several Figures:

FIGS. 1 through 8 are a series of cross sectional views illustrating amethod of forming a back gate ETSOI device in accordance with anexemplary embodiment, in which:

FIG. 1 illustrates the formation of a protective pad layer over an ETSOIsubstrate;

FIG. 2 illustrates the formation of STI trenches within the substrate ofFIG. 1 and the formation of sidewall spacers within the STI trenches;

FIG. 3 illustrates filling of the STI trenches of FIG. 2 with a dopantsource insulator material;

FIG. 4 illustrates optional recessing of a portion of the dopant sourceinsulator material, and replacement with an undoped insulator material;

FIG. 5 illustrates a thermal anneal of the structure of FIG. 4,resulting in the formation of a doped well region in the bulk substratelayer beneath the BOX layer;

FIG. 6 illustrates planarization of the structure of FIG. 5 to removethe protective pad layer;

FIG. 7 illustrates an expanded view of the structure of FIG. 6,including the formation of an adjacent doped well region of an oppositepolarity type;

FIG. 8 illustrates the formation of back gate transistor devices usingthe structure of FIG. 7;

FIGS. 9 through 12 are a series of cross sectional views illustrating amethod of forming a back gate ETSOI device in accordance with anotherexemplary embodiment, in which:

FIG. 9 illustrates the formation of STI trenches within the substrate ofFIG. 1 and the formation of sidewall spacers within the STI trenches;

FIG. 10 illustrates the formation of a doped well in the bulk substratelayer beneath the BOX layer of FIG. 9;

FIG. 11 illustrates an expanded view of the structure of FIG. 10,including the presence of a protective layer over an adjacent region ofthe substrate for which a doped well region of an opposite polarity typeis to be formed;

FIG. 12 illustrates the filling of deep trench isolation regions andshallow trench isolation regions of the structure of FIG. 11 with a sameinsulator material.

DETAILED DESCRIPTION

Ultrathin body MOSFETs, such as ETSOI (extremely thin SOI) devices, areconsidered viable options for CMOS scaling for the 22 nanometer (nm)node and beyond. ETSOI with a back gate bias (as described above) hasbeen touted as a potential device option for future CMOS. In such astructure, the back bias is implemented by a back gate that comprises adoped region below the buried oxide (BOX) layer. Typically, the dopedregion defining the back gate is formed by dopant implantation directlythrough the ETSOI layer, the BOX layer, and into the bulk substrate.However, such an implantation approach has several drawbacks. First,implantation straggling may create undesired residual dopants in theactive SOI layer, which adversely increases device variability. Second,the implantation into the BOX layer may cause defects therein andincrease the etch rate of the BOX layer during device fabrication, inturn creating potential shorts between the SOI layer above the BOX layerand the bulk substrate below the BOX layer.

Accordingly, disclosed herein are methods and corresponding structuresfor forming ETSOI back gate regions without the problems describedabove. In one embodiment, the back gate region is formed by filling anopen STI trench with a doped material and diffusing the dopants from thedoped STI into the bulk substrate. In another embodiment, dopants may bedirectly implanted into the bulk substrate through the open STI trench.

Referring initially to FIGS. 1 through 8, there are shown a series ofcross sectional views a method of forming a back gate ETSOI device inaccordance with an exemplary embodiment.

As shown in FIG. 1, an ETSOI substrate includes a bulk semiconductorlayer 102, a buried insulator (e.g., oxide) (BOX) layer 104 formed onthe bulk semiconductor layer 102, and an active ETSOI layer 106 formedon the BOX layer 104. A protective pad layer 108 (e.g., oxide, nitride,etc.) is also shown formed over the ETSOI layer 106. The semiconductorsubstrate material of the bulk layer 102 and/or the ETSOI layer 106 maybe selected from, but is not limited to, silicon, germanium,silicon-germanium alloy, silicon carbon alloy, silicon-germanium-carbonalloy, gallium arsenide, indium arsenide, indium phosphide, III-Vcompound semiconductor materials, II-VI compound semiconductormaterials, organic semiconductor materials, and other compoundsemiconductor materials.

In addition, while the exemplary embodiments are described in terms ofan ETSOI thickness device (e.g., the ETSOI layer 106 is on the order ofabout 10 nanometers (nm) or less), it should be appreciated that theprinciples described herein are equally applicable to substrates ofvarying materials and thicknesses.

FIG. 2 illustrates the formation of STI trenches 110 within thesubstrate of FIG. 1. Specifically, the STI trenches 110 are definedthrough the protective pad layer 108, the ETSOI layer 106, the BOX layer104, and slightly into a portion of the bulk semiconductor layer 102.The STI trenches 110 may be formed, for example, by photolithographicpatterning and successive etching of the layers. As also shown in FIG.2, the STI trenches 110 are further provided with sidewall spacers 112,such as a nitride material for example.

Following the spacer formation, FIG. 3 illustrates filling of the STItrenches 110 with a dopant source insulator material 114, which materialwill provide the source of dopant atoms for the back gate region. Thespecific selection for the dopant source insulator material 114 dependson whether the desired polarity of the back gate device is n-type orp-type. Suitable examples of the dopant source insulator material 114include, but are not limited to, boron-doped oxide, arsenic-doped oxide,and phosphorous-doped oxide. Because the dopant materials are intendedto be located within the bulk semiconductor layer 102, the entire heightof the STI trenches 110 need not be completely filled with the dopantsource insulator material 114. Accordingly, FIG. 4 illustrates anoptional recessing of a portion of the dopant source insulator material114, and replacement of the same with an undoped insulator material 116such as oxide for example. In the example depicted, the dopant sourceinsulator material 114 may be recessed below the bottom surface of theETSOI layer 106. In an exemplary embodiment, the undoped insulatormaterial 116 may be formed by a high density plasma deposition, followedby chemical mechanical planarization/polishing (CMP).

Referring now to FIG. 5, a thermal anneal of the structure of FIG. 4 isperformed so as to result in the formation of a doped well region 118 inthe bulk substrate 102 layer beneath the BOX layer 104. Notably, thepresence of the sidewall spacers 112 (and optional undoped insulatormaterial 116) prevents dopant diffusion from the dopant source insulatormaterial 114 into the ETSOI layer 106 or the BOX layer 104. Once thestructure is planarized to remove the protective pad layer 108 as shownin FIG. 6, device processing may continue to form back gate devices. Incontrast to the aforementioned technique of implanting a doped wellregion directly through SOI and BOX layers, here the contamination ofthe ETSOI layer 106 and the BOX layer 104 is prevented by using the STItrenches to deliver the dopant material into the bulk substrate 102layer.

By way of further illustration, FIG. 7 is an expanded view of thestructure of FIG. 6, which also depicts the formation of an adjacentdoped well region of an opposite polarity type and/or dopantconcentration. In this specific example, region 120 represents a p-typeregion wherein the doped well region 118 is a p-well region. Here, thedopant source insulator material 114 may include a boron-doped oxide forexample. Conversely, region 122 represents an adjacent p-type regionwherein the doped well region 118′ is an n-well region. In this case,the dopant source insulator material 114′ may include an arsenic-dopedoxide or a phosphorous-doped oxide for example. As further shown in FIG.7, adjacent n-wells and p-wells are isolated from one another by deeptrench isolation regions 124. FIG. 8 then illustrates the formation ofback gate transistor devices 126 using the structure of FIG. 7.

In lieu of leaving a dopant source insulator material in at least aportion of the STI regions, other doping techniques are alsocontemplated with respect to using the open STI trenches. In thisregard, FIGS. 9 through 12 are a series of cross sectional viewsillustrating a method of forming a back gate ETSOI device in accordancewith another exemplary embodiment. For ease of description, likeelements are identified with similar reference numbers in thisembodiment.

FIG. 9 illustrates the formation of STI trenches 110 within thesubstrate of FIG. 1 and the formation of sidewall spacers 112 within theSTI trenches 110, similar to the view presented in FIG. 2. In FIG. 10, adoped well 118 is formed in the bulk substrate layer beneath the BOXlayer of FIG. 9. Notably, no dopant source insulator material is presentin the STI trenches 110 after the doped well formation. In oneembodiment, this may be implemented by a doping technique (indicated bythe arrows) such as, for example, plasma doping, plasma immersiondoping, and gas phase doping, etc. In another embodiment, the dopantsource insulator and anneal technique of FIGS. 3-5 could also be used.However, instead of leaving the dopant source insulator material in thetrenches 110, the doped material may then be stripped so as to result inthe intermediate structure illustrated in FIG. 10.

Referring now to FIG. 11, there is shown an expanded view of thestructure of FIG. 10, including the presence of a protective layer 126over an adjacent region of the substrate for which a doped well regionof an opposite polarity type is to be formed. For example, during theformation of the doped well 118 in region 120 of the device, theprotective layer 126 (e.g., a hardmask) remains over region 122 of thedevice during formation of the spacers 112 so that only the STI bottomscorresponding to locations where the well regions are to be formed areexposed. Then, to form a corresponding well in region 122, theprotective layer 126 is removed and another protective layer (not shown)is selectively formed over the trenches 110 in region 120 so that acorresponding spacer and doping process may be performed for region 122.

Finally, as illustrated in FIG. 12, once complementary polarity wellregions 118 and 118′ have been formed, all of the trenches (includingshallow and deep trench isolation regions) are filled with a sameinsulator material, such as an oxide for example. As will thus beappreciated, by performing back gate doping following STI definition,the above described embodiments avoid previous issues of BOX and ETSOIdopant defects.

While the invention has been described with reference to an exemplaryembodiment or embodiments, it will be understood by those skilled in theart that various changes may be made and equivalents may be substitutedfor elements thereof without departing from the scope of the invention.In addition, many modifications may be made to adapt a particularsituation or material to the teachings of the invention withoutdeparting from the essential scope thereof. Therefore, it is intendedthat the invention not be limited to the particular embodiment disclosedas the best mode contemplated for carrying out this invention, but thatthe invention will include all embodiments falling within the scope ofthe appended claims.

1. A method of forming a back gate transistor device, the methodcomprising: forming an open isolation trench in a substrate; formingsidewall spacers in the open isolation trench; and using the openisolation trench to perform a doping operation so as to define a dopedwell region below a bottom surface of the isolation trench that servesas a back gate conductor, wherein the sidewall spacers preventcontamination of a channel region of the back gate transistor device bydopants; wherein the performing the doping operation further comprisesforming a dopant source insulator material within the open isolationtrench, performing an annealing operation so as to drive dopants fromthe dopant source insulator material into the substrate, and performingone of: recessing a portion of the dopant source insulator material andreplacing the same with an undoped insulator material prior toperforming the annealing operation; or completely removing the dopantsource insulator material following the annealing operation. 2-5.(canceled)
 6. The method of claim 1, wherein the doped well regioncomprises a p-type region, and the dopant source insulator materialcomprises boron-doped oxide.
 7. The method of claim 1, wherein the dopedwell region comprises an n-type region, and the dopant source insulatormaterial comprises one or more of arsenic-doped oxide andphosphorous-doped oxide.
 8. The method of claim 1, wherein the substratecomprises a semiconductor-on-insulator (SOI) substrate having a bulksubstrate layer, a buried insulator (BOX) layer formed on the bulksubstrate layer, and an active SOI layer formed on the buried insulatorlayer.
 9. The method of claim 8, wherein the SOI substrate comprises anextremely thin SOI (ETSOI) substrate, with the active SOI layer having athickness of about 10 nanometers (nm) or less.
 10. The method of claim8, wherein the open isolation trench is formed completely through theactive SOI layer and the BOX layer, and into a portion of the bulksubstrate layer such that the sidewall spacers prevent contamination ofthe active SOI layer and the BOX layer by the dopants.
 11. A method offorming a back gate transistor device, the method comprising: forming aprotective pad layer over a semiconductor-on-insulator (SOI) substrate,the SOI substrate having a bulk substrate layer, a buried insulator(BOX) layer formed on the bulk substrate layer, and an active SOI layerformed on the buried insulator layer; forming an open isolation trenchthrough the protective pad layer, the active SOI layer, the BOX layer,and into a portion of the bulk substrate layer; forming sidewall spacersin the open isolation trench; and using the open isolation trench toperform a doping operation so as to define a doped well region below abottom surface of the isolation trench and the BOX layer, such that thedoped well region serves as a back gate conductor, wherein the sidewallspacers prevent contamination of the BOX layer and the active SOI layerdevice by dopants; wherein the performing the doping operation furthercomprises forming a dopant source insulator material within the openisolation trench, performing an annealing operation so as to drivedopants from the dopant source insulator material into the bulksubstrate layer, and performing one of: recessing a portion of thedopant source insulator material and replacing the same with an undopedinsulator material prior to performing the annealing operation; orcompletely removing the dopant source insulator material following theannealing operation. 12-14. (canceled)
 15. The method of claim 11,further comprising refilling the open isolation trench with an undopedinsulator material.
 16. The method of claim 11, wherein the doped wellregion comprises a p-type region, and the dopant source insulatormaterial comprises boron-doped oxide.
 17. The method of claim 11,wherein the doped well region comprises an n-type region, and the dopantsource insulator material comprises one or more of arsenic-doped oxideand phosphorous-doped oxide.
 18. (canceled)
 19. (canceled)
 20. Themethod of claim 11, wherein the SOI substrate comprises an extremelythin SOI (ETSOI) substrate, with the active SOI layer having a thicknessof about 10 nanometers (nm) or less. 21-25. (canceled)